Sugar and organic acid utilisation in Pachysolen tannophilus and Pichia stipitis : a thesis presented in partial fulfilment of the requirements for the degree in Master of Science in Genetics at Massey University

Abstract

Wild-type strains of the K+ yeasts Pachysolen tannophilus and Pichia stipitis were assessed for their ability to utilise tricarboxylic acid (TCA) cycle intermediates and to ferment glucose and xylose. These two species of yeast were mutagenised using ultraviolet (UV) radiation. The survivors were screened on an indicator plate for mutants with altered rates of malate utilisation, in the presence of glucose. The Pichia stipitis wild-type strain was UV mutagenised, and mutants resistant to the lethal analog of glucose, 2-deoxyglucose, were selected.

These mutants, plus several mutants of Pachysolen tannophilus isolated in an earlier study, were partially characterised by growth on a variety of solid media. The mutants were also grown on indicator media containing xylose or glucose plus one TCA cycle intermediate: citrate, succinate, fumarate or L-malate. Experiments with the indicator media served both as a method of characterisation of the mutants, and as a method of testing the utility of the indicator plate. The behavior of these mutants when grown under semi-anaerobic conditions in mixtures of glucose and TCA cycle intermediates was investigated. The wild-type Pachysolen tannophilus co-utilised malate and glucose, whereas in the wild-type Pichia stipitis, malate utilisation was repressed by glucose. Some mutants of both species showed increased utilisation of malate (and succinate, where tested). In Pachysolen tannophilus, this behavior correlated with loss of all three hexose-ATP-kinase enzymes, and in Pichia stipitis the behavior correlated with the loss of any hexose­ ATP-kinase activity.